Get some random water blasters, plastic cups, water pistols, hoses, buckets, kiddie pools, etc. and set them loose. (Water balloons are not a good idea- the bits of latex left behind are bad for animals).
You may even be able to get away with this in a city park but may have to bring your own buckets of water if there isn't a water fountain. Just be aware of local rules. I know it would be fine in the park near where my sister lived in Cambridge, MA or in the park near my other sister in San Francisco. Other parks may be more restrictive.
These photos were taken on a rainy 65 degree day a couple of weeks ago. They don't care if it's cold!